Audio signals, like speech or music, are encoded for example to enable efficient transmission or storage of the audio signals.
Audio encoders and decoders are used to represent audio based signals, such as music and background noise. These types of coders typically do not utilise a speech model for the coding process, rather they use processes for representing all types of audio signals, including speech.
Speech encoders and decoders (codecs) are usually optimised for speech signals, and can operate at either a fixed or variable bit rate.
Coefficients from speech and audio codecs may be quantised using vector quantisation techniques. In particular, some coefficients can be quantised using predictive quantisation whereby past quantised vectors of are used to predict the value of a current vector of coefficients.
However, the use of prediction for quantising vectors can result in annoying artefacts in the decoded speech or audio signal. This can particularly occur during a frame erasure event, especially after a long run of prediction in the vector quantiser.